Water splitted from the overflow coolant

[alcubot]

Guys I almost signed for a 986 S in 1999 with 108k km.
Price 11k euros.
The car looks very well put, ok interior, hood ok, ok paint, the engine runs very well.
The only thing I noticed: after making a nice ride over 25 km the 986 S has "spit" from the overflow coolant some water.
I touched it and it was not oiled, I checked the oil and there is NOT white foam.
I read of users who have had problems with the expansion tank, even the cap ....
The seller claims that when he put the water has simply inserted more than expected ...
What could be in your opinion?
Thanks in advance.
Antonio